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    <title>Document</title>
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<body>
    <script>
        console.log('hehe'-10)  //NaN
        console.log('100'-10)   //90
        console.log('100'+10)   //10010
        console.log(10+"100")   //10100
    </script>
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    -只能数值操作，所以是将‘100'字符串转为100数值
    +不光数值可以操作，字符串也可以拼接,把10转为'10'在拼接
